One example of conventional vapor compression refrigeration systems provided with a compression mechanism having a plurality of compressors are air conditioning systems used to air-condition buildings. This kind of air conditioning system is provided with a plurality of user units and a heat source unit with a large capacity that is sufficient for accommodating the heating and cooling loads of the user units. In order to enable the system to be operated in a partial load mode, the heat source unit is provided with a compression mechanism made up of a plurality of comparatively small-capacity compressors connected in parallel. The compression mechanism is provided with an oil equalizing circuit including an oil separator connected to the discharge sides of the compressors, oil return pipes for returning the oil separated by the oil separator to the compressors, and oil equalizing pipes connected between the compressors for reducing imbalances in the amount of oil in the compressors.
In the conventional compression mechanism just described, the oil equalizing circuit around the compressors becomes complex because it includes a return pipe for each compressor and a plurality of equalizing pipes connected between the compressors. The larger the number of compressors, the more complex the oil equalizing circuit becomes.
In a system whose compression mechanism has three or more compressors, a plurality of combinations of running compressors and stopped compressors occur when the system is operated in partial load mode and it is difficult to supply sufficient oil to the running compressors during all of the operating combinations.